Method and apparatus for electroplating

ABSTRACT

An apparatus for electroplating a layer of metal onto the surface of a wafer includes an ionically resistive ionically permeable element located in close proximity of the wafer and an auxiliary cathode located between the anode and the ionically resistive ionically permeable element. The ionically resistive ionically permeable element serves to modulate ionic current at the wafer surface. The auxiliary cathode is configured to shape the current distribution from the anode. The provided configuration effectively redistributes ionic current in the plating system allowing plating of uniform metal layers and mitigating the terminal effect.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in part of U.S. application Ser. No.12/291,356, filed Nov. 7, 2008, now issued as U.S. Pat. No. 8,308,931,naming Reid et al. as inventors, which is herein incorporated byreference in its entirety and for all purposes.

BACKGROUND OF THE INVENTION

The transition from aluminum to copper in integrated circuit (IC)fabrication required a change in process “architecture” (to damasceneand dual-damascene) as well as a whole new set of process technologies.One process step used in producing copper damascene circuits is theformation of a “seed-” or “strike-” layer, which is then used as a baselayer onto which copper is electroplated (electrofill). The seed layercarries the electrical plating current from the edge region of the wafer(where electrical contact is made) to all trench and via structureslocated across the wafer surface. The seed film is typically a thinconductive copper layer. It is separated from the insulating silicondioxide or other dielectric by a barrier layer. The use of thin seedlayers (which may also act simultaneously as copper diffusion barrierlayers) which are either alloys of copper or other metals, such asruthenium or tantalum, has also been investigated. More detail on suchseed layers can be found in U.S. patent application Ser. No. 12/359,997,entitled Diffusion Barrier Layers, filed Jan. 26, 2009, which isincorporated herein by reference. The seed layer deposition processshould yield a layer which has good overall adhesion, excellent stepcoverage (more particularly, conformal/continuous amounts of metaldeposited onto the side-walls of an embedded structure), and minimalclosure or “necking” of the top of the embedded feature.

Market trends of increasingly smaller features and alternative seedingprocesses drive the need for a capability to plate with a high degree ofuniformity on increasingly thin seeded wafers. In the future, it isanticipated that the seed film may simply be composed of a plateablebarrier film, such as ruthenium, a bilayer of a very thin barrier andcopper (deposited, for example, by an atomic layer deposition (ALD) orsimilar process), or an alloy of various metals. These thin films, somehaving inherently large specific resistivities, present the engineerwith an extreme terminal effect situation. For example, when driving a 3amp total current uniformly into a 30 ohm per square ruthenium seedlayer (a likely value for a 30-50 Å film), the resultant center to edgevoltage drop in the metal will be over 2 volts. To effectively plate alarge surface area, the plating tooling makes electrical contact to theconductive seed only in the edge region of the wafer substrate. There isno direct contact made to the central region of the substrate. Hence,for highly resistive seed layers, the potential at the edge of the layeris significantly greater than at the central region of the layer.Without appropriate means of resistance and voltage compensation, thislarge edge-to-center voltage drop could lead to an extremely non-uniformplating thickness distribution, primarily characterized by thickerplating at the wafer edge. For comparison, the thermodynamic limit ofthe voltage drop for electrolyte solvent (water) is only about 1.4V.

FIG. 1 is a schematic of an approximated equivalent electrical circuitfor the problem. It is simplified to one dimension for clarity. Thecontinuous resistance in the seed layer is represented by a set offinite (in this case four) parallel circuit elements. The in-filmresistor elements R_(f), represent the differential resistance from anouter radial point to a more central radial point on the wafer. Thetotal current supplied at the edge, I_(t), is distributed to the varioussurface elements, I₁, I₂, etc., scaled by the total path resistanceswith respect to all the other resistances. The circuits more centrallylocated have a larger total resistance because of thecumulative/additive resistance of the R_(f) for those paths.Mathematically, the fractional current F_(i) through any one of thesurface element paths is

$\begin{matrix}\begin{matrix}{F_{i} = \frac{I_{i}}{I_{t}}} \\{= \frac{Z_{T}}{Z_{i}}} \\{= \frac{\frac{1}{\left( {{iR}_{f} + R_{{ct},i} + {Zw}_{i} + R_{{el},i}} \right)}}{\sum\limits_{1}^{n}\frac{1}{{iR}_{f} + R_{{ct},i} + {Zw}_{i} + R_{{el},i}}}}\end{matrix} & (1)\end{matrix}$where n is the total number of parallel paths that the circuit isdivided into, i (sometime used as a subscript) refers to the i^(th)parallel current path (from the edge terminal), t refers to the totalcircuit, I is current, R_(f) is the resistance in the metal film betweeneach element (constructed, for simplicity, to be the same between eachadjacent element), R_(ct) is the local charge transfer resistance, Z_(w)is the local diffusion (or Warberg) impedance and R_(el) is theelectrolyte resistance. With this, I_(i) is the current to through thei^(th) surface element pathway, and I_(t) is the total current to thewafer. The charge transfer resistance at each interfacial location isrepresented by a set of resistors R_(ct) in parallel with the doublelayer capacitance C_(dl), but for the steady state case does not effectthe current distribution. The diffusion resistances, represented by theWarberg impedance (symbol Z_(w)) and the electrolyte resistance (R_(el))are shown in a set of parallel circuit paths, all in series with theparticular surface element circuit, give one of several parallel pathsfor the current to traverse to the anode. In practice, R_(ct) and Z_(w)are quite non-linear (depending on current, time, concentrations, etc.),but this fact does not diminish the utility of this model in comparinghow the current art and this disclosure differ in accomplishing uniformcurrent distribution. To achieve a substantially uniform currentdistribution, the fractional current should be the same, irrespective ofthe element position (i). When all terms other than the film resistanceterm, R_(f), are relatively small, the current to the i^(th) element is

$\begin{matrix}{F = \frac{\frac{1}{i}}{\sum\limits_{1}^{n}\frac{1}{i}}} & (2)\end{matrix}$

Equation 2 has a strong i (location) dependence and results when nosignificant current distribution compensating effects are active. In theother extreme, when R_(ct), Z_(w), R_(el) or the sum of these terms aregreater than R_(f), the fractional current approaches a uniformdistribution; the limit of equation 1 as these parameters become largeis F=1/n, independent of location i.

Classical means of improving plating non-uniformity draw upon (1)increase R_(ct) through the use of copper complexing agents or chargetransfer inhibitors (e.g., plating suppressors and levelers, with thegoal of creating a large normal-to-the-surface voltage drop, makingR_(f) small with respect to R_(ct)), (2) very high ionic electrolyteresistances (yielding a similar effect through R_(el)), (3) creating asignificant diffusion resistance (Z_(w)), or (4) variations of a platingcurrent recipe to minimize voltage drop, or control of mass transferrate to limit current density in areas of high interfacial voltage drop(see e.g., U.S. Pat. Nos. 6,110,344, 6,074,544, and 6,162,344, each ofwhich is incorporated herein by reference).

These approaches have significant limitations related to the physicalproperties of the materials and the processes. Typical surfacepolarization derived by organic additives cannot create polarization inexcess of about 0.5V (which is a relatively small value in comparisonto, for example, the 2V seed layer voltage drop that must be compensatedas noted above). Also, because the conductivity of a plating bath istied to its ionic concentration and pH, decreasing the conductivitydirectly and negatively impacts the rate of plating and morphology ofthe plated material.

What is needed therefore is an improved technique for uniformelectroplating onto thin-metal seeded wafers, particularly wafers withlarge diameters (e.g. 300 mm).

SUMMARY

These needs are addressed, in one aspect, by providing an electroplatingapparatus and a method for uniform electroplating that make use of anionically resistive element having electrolyte-permeable pores or holes,where the element resides in close proximity of the wafer substrate. Theionically resistive ionically permeable element described hereinsubstantially improves plating uniformity on thin resistive seed layers.It is particularly useful when employed in combination with an auxiliarycathode configured to divert or remove a portion of current from theanode that would otherwise pass to the edge region of the wafer. Incertain embodiments, the auxiliary cathode resides between the ionicallyresistive ionically permeable element and an anode in the electroplatingapparatus. In certain embodiments, the auxiliary cathode is located inthis position as a virtual cathode. The ionically resistive ionicallypermeable element described herein presents a uniform current density inthe proximity of the wafer cathode and therefore serves as a virtualanode. Accordingly, the ionically resistive ionically permeable elementwill be also referred to as a high-resistance virtual anode (HRVA)

In certain embodiments, the HRVA is located in close proximity to thewafer. In certain embodiments, the HRVA contains a plurality ofthrough-holes that are isolated from each other and do not forminterconnecting channels within the body of HRVA. Such through-holeswill be referred to as 1-D through-holes because they extend in onedimension, typically, but not necessarily, normal to the plated surfaceof the wafer. These through-holes are distinct from three-dimensionalporous networks, where the channels extend in three dimensions and forminterconnecting pore structures. An example of a HRVA is a disk made ofan ionically resistive material, such as polycarbonate, polyethylene,polypropylene, polyvinylidene diflouride (PVDF),polytetrafluoroethylene, polysulphone and the like, having between about6,000-12,000 1-D through-holes. In certain embodiments, the HRVA is aporous structure in which at least some of the pores are interconnectedand therefore allow some two- or three-dimensional movement ofelectrolyte therein. The disk, in many embodiments, is substantiallycoextensive with the wafer (e.g., has a diameter of about 300 mm whenused with a 300 mm wafer) and resides in close proximity of the wafer,e.g., just below the wafer in a wafer-facing-down electroplatingapparatus. In some embodiments, the disk is relatively thin, for examplebetween about 5 and 50 mm thick. The plating electrolyte containedwithin the pores of the HRVA allows ionic current to pass though thedisk, but at a significant voltage drop compared to the system as awhole. For example, the voltage drop in the HRVA may be greater thanabout 50%, for example, between about 55 and 95%, of the total voltagedrop between the counter electrode (anode) and the wafer peripheraledge. In certain embodiments, the plated surface of the wafer resideswithin about 10 mm, and in some embodiments, within about 5 mm, of theclosest HRVA surface.

In an embodiment of an apparatus for electroplating metal onto asubstrate, the apparatus includes a plating chamber configured tocontain an electrolyte and an anode while electroplating metal onto thesubstrate A substrate holder is configured to hold the substrate suchthat a plating face of the substrate is positioned at a defined distancefrom the anode during electroplating, the substrate holder having one ormore electrical power contacts arranged to contact an edge of thesubstrate and provide electrical current to the substrate duringelectroplating. An ionically resistive ionically permeable element ispositioned between the substrate and the anode during electroplating,the ionically resistive ionically permeable element having a flatsurface that is substantially parallel to and separated from a platingface of the substrate by a gap of about 5 millimeters or less duringelectroplating. In some embodiments, the anode may be located in acurrent-confining and directing anode-chamber that allows current tosubstantially only exit the chamber though the pores of the ionicallyresistive ionically permeable element, and an auxiliary cathode, locatedbetween the anode and the ionically resistive ionically permeableelement, and peripherally oriented to shape the current distributionfrom the anode while the auxiliary cathode is supplied with currentduring electroplating.

In another embodiment of the invention, a metal layer is plated onto asubstrate. Plating the metal layer includes: (a) holding a substrate,having a conductive seed and/or barrier layer disposed on its surface,in a substrate holder of an electroplating apparatus; (b) immersing aworking surface of the substrate in an electrolyte solution andproximate an ionically resistive ionically permeable element positionedbetween the working surface and the anode contained in the platingchamber, and in some embodiments, the anode may be located in acurrent-confining and directing anode-chamber that allows current tosubstantially only exit the chamber though the pores of the ionicallyresistive ionically permeable element, the ionically resistive ionicallypermeable element having a flat surface that is parallel to andseparated from a working face of the substrate by a gap of about 5millimeters or less; (c) supplying current to the substrate to plate themetal layer onto the seed and/or barrier layer; and (d) supplyingcurrent to an auxiliary cathode located between the anode and theionically resistive ionically permeable element to shape the currentdistribution from the anode.

These and other features and advantages of the present invention will bedescribed in more detail below with reference to the associateddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic diagram depicting an equivalent circuit forelectroplating on a thin seed layer.

FIG. 1B is a schematic diagram depicting an equivalent circuit forelectroplating on a thin seed layer in a presence of an ionicallyresistive ionically permeable element having 3-D porous network.

FIG. 1C is a schematic diagram depicting an equivalent circuit forelectroplating on a thin seed layer in a presence of an ionicallyresistive ionically permeable element having 1-D porous network

FIG. 2A is a schematic representation of a top view of an ionicallyresistive element having a plurality of 1-D through-holes, in accordancewith embodiments presented herein.

FIG. 2B is a schematic representation of a cross-sectional view of theionically resistive element having a plurality of 1-D through-holes, inaccordance with embodiments presented herein.

FIGS. 3A-3D are cross-sectional schematic views emphasizing differentcomponents of a representative electroplating apparatus in accordancewith embodiments presented herein.

FIG. 4 is a cross-sectional view of the top portion of electroplatingapparatus illustrating a wafer, a HRVA, and a second auxiliary cathode.

FIG. 5 is a cross-sectional view of the top portion of electroplatingapparatus illustrating a wafer, a HRVA, a second auxiliary cathode, anda stationary shield positioned above HRVA and on the periphery of HRVA.

FIG. 6A-6D are cross-sectional schematic views of four differentelectroplating apparatus configurations. Current and voltage lines areillustrated on the cross-sectional schematics. Also shown arerepresentative plots of current density versus radial position for eachof the electroplating apparatuses.

FIG. 7 is a process flow diagram for an electroplating apparatusincorporating a HRVA, an auxiliary cathode, and a second auxiliarycathode, in accordance with an embodiment presented herein.

FIG. 8 is a plot showing the post-plating sheet resistances andcalculated film thicknesses of wafers, having initial sheet resistancesof 50 ohm per square, plated with copper.

FIG. 9 is a plot showing the post-plating sheet resistances of wafers,having initial sheet resistances of 10 ohm per square, plated withcopper.

FIG. 10 is a plot showing the pre-plating sheet resistance and apost-plating sheet resistance of a wafer plated using embodiments of thedisclosed invention.

FIG. 11 is a finite element mesh used for numerical simulations ofembodiments of the present invention.

FIG. 12 is a plot of the results of finite element modeling, showingcurrent density versus radial location.

DETAILED DESCRIPTION

Advanced technologies call for the electroplating of metals onto waferswith sheet resistances of 10 ohm per square and higher (even 20 ohms persquare or 40 ohms per square or higher). This requires ever moreaggressive techniques (i.e., techniques other than only the use of aHRVA alone or a thief electrode alone) to compensate for the terminaleffect. During plating, the thickness of metal and the sheet resistancecan drop several orders of magnitude in a short time, and so methods andapparatus capable of plating uniformly on the wafer throughout a processwhere there may be a rapidly initially varying and later a relativelyconstant sheet resistance are required. Embodiments of the presentinvention address the challenges presented by such high resistance seedlayers, the rapid dynamic variance in the seed electrical parameters,and the extreme terminal effect they present.

Embodiments of the present invention pertain to methods and apparatusesfor electroplating a substantially uniform layer of metal onto a workpiece having a seed layer thereon. In certain embodiments, a platingcell includes both a porous HRVA in close proximity to a work piece anda thief electrode (referred to also as an “auxiliary” cathode). The“thief” electrode is located between the HRVA and an anode. In somecases, multiple thief electrodes may be used. A thief cathode can removeor divert a portion of ionic current emanating from the anode which isdirected to the outer periphery, including the very edge of the wafer inthe absence of the thief cathode, thereby modifying the current andenabling a vastly improved uniformity of current density experienced bythe wafer. In some cases, however, it may be desirable to useembodiments of the invention to create a non-uniform current densitythat is experienced by the wafer. For example, it may be desirable tocreate a non-uniform current density, resulting in non-uniform metalplating, during overburden deposition to aid in chemical mechanicalpolishing (CMP), wet chemical etching, electropolishing, orelectromechanical polishing.

Significantly, the use of the auxiliary cathode in combination with aporous HRVA positioned in close proximity of the wafer providesadvantages well in excess of the advantages provided individually bythese elements, and the combination of the two work in a synergisticmanner. The auxiliary cathode is positioned between the plating cell'sanode and HRVA. In the case of a vertically oriented cell, the auxiliarycathode is located below the HRVA. In certain embodiments, the auxiliarycathode is generally ring or annularly shaped to provide significantimpact on the current density distribution at the peripheral region ofthe work piece. The advantages of this cathode may be accentuated by asmall HRVA-to-wafer spacing, and/or by restricting the flow of currentin the body of the HRVA.

One of the advantages of employing an auxiliary cathode for modulatingthe current directed at a wafer (over, for example, a moving mechanicalshield or iris) is that the level of current applied to the auxiliarycathode can be rapidly and dynamically controlled during the platingprocess (e.g., times shorter than a few seconds) to account for rapidlychanging metal sheet resistance as the metal is deposited. This aids inkeeping the plating non-uniformity to a minimum during different timesin the plating process. For example, the level of current applied to theauxiliary cathode can start at high level when the layer is thin, andthen can be gradually or incrementally reduced during plating (e.g.,over a period of a few seconds) as the thickness of the plated layerincreases and the severity of the terminal effect subsides.

A HRVA and/or a second auxiliary cathode, positioned near the workpiece, can influence the plating surface of the work piece and reshapethe current distribution on a wafer by changing the voltage and currentdistribution only in a region in close proximity to the face of the workpiece. These elements do not significantly impact the currentdistribution within the electrolyte or at the anode at a significantdistance from the work piece surface, such as below the HRVA. Thus,these measures (using the HRVA and/or second auxiliary cathode locatednear the wafer or HRVA as described herein) have little or no impact onthe current distribution closer to the anode which resides below theHRVA. In many cases, the ionic current distribution remains nearlyconstant in the region between the anode and the HRVA.

The HRVA alone generally will improve the long range radial currentdistribution over configurations without a HRVA (from less uniform tomore uniform). However, without a specific radial-pore-pattern limitedto the application over a thickness/sheet resistance range or amechanically activated dynamic change in shielding, the radial currentdistribution generally tends to be less than perfectly uniform,generally center thin. A secondary auxiliary cathode, positioned abovethe HRVA and peripheral to the wafer edge, can dynamically influence theedge current distribution (typically limited to a region within about1-3 cm from the edge), but not change the central plating region'scurrent distribution. For some applications, particularly situationswhere the sheet resistance is exceedingly large, using a HRVA and/orsecond auxiliary cathode as described herein may be insufficient tofully overcome the terminal effect.

It may be necessary to modify the current distribution inside theelectrolyte at positions well removed from the work piece, i.e., at aposition relatively closer to the anode, to adequately address theterminal effect when very high resistance seed or seed/barriercombination layers are used. In certain embodiments described herein,this is accomplished by positioning an auxiliary cathode at a locationbelow the HRVA and between the work piece and the anode. The auxiliarycathode is shaped and oriented to modify the current densitydistribution within the electrolyte in a plane parallel to the wafer,below the HRVA, and located some distance from the work piece in amanner that reduces the current density and current vector (flowdirection) in regions of the plane below and corresponding to the edgeregions of the work piece. This is similar to the on-wafer effect of aphysical iris or shield placed below a work piece in a plating chamber.For this reason, the auxiliary electrodes of embodiments of thisinvention are sometimes referred to as “electronic irises”, or an“EIRIS”, because an electronic auxiliary electrode is used to accomplisha result similar to that of a physical iris placed in the current pathbetween the wafer and the anode. In the case an EIRIS, however, thecurrent vector trajectory is shifted radially outwards, rather thanbeing blocked at larger radii and being forced and squeezed inwards witha physical iris.

To elaborate, one difference between an EIRIS and a physical iris orshield is that all the current from the anode passes through thephysical iris, as it “squeezes through” the iris or shieldingrestriction. Current is largely or completely blocked by the iris shieldand is re-routed from the edge regions radially inwards before passingupwards. As a result, the central current density in the region of theshield opening is generally increased. In the case of the EIRIS, not allthe current emanating from the anode arrives at the wafer, as some ofthe edge current is generally diverted radially outwards towards theauxiliary electrode. Above the auxiliary cathode the magnitude of thecurrent density vector directed at the wafer tends to be reduced becauseof the diversion, but the current density in the central region of anEIRIS-equipped electroplating apparatus above the EIRIS is only slightlydecreased or perhaps unaltered vs. the non-EIRIS case.

The region where the auxiliary cathode acts is generally parallel to thesubstrate surface and separated therefrom. Generally, it is desirable tohave the auxiliary cathode located relatively close to the lower surfaceof the HRVA so that the current does not have the space in which toredistribute to a more non-uniform profile before reaching the HRVAsurface. The distance, d, between the lower surface of the HRVA and theauxiliary cathode should generally be approximately equal to or lessthan the radius, r, of wafer onto which metal is being plated (i.e., d˜≦r). The auxiliary cathode should also be significantly above the planeof the anode so the current from the anode has space to changedirections without unduly large auxiliary cathode voltages or currents.

Generally, the distance of the auxiliary cathode in the anode chamberand below the wafer and HRVA (when the system has a HRVA) should be keptto less than about 50% of the wafer diameter. For example, for a 300 mmwafer, the auxiliary cathode might be between about 0.75 to 6.5 inchesbelow the wafer and between about 0.25 and 6 inches below a HRVA. Incontrast, the location of the anode relative to the wafer, HRVA (whenemployed), and auxiliary cathode is a compromise between functionalperformance as well as engineering waste. Typically, the anode shouldgenerally be in the anode chamber and below all three of these elements.But while the electroplating apparatus might have the anode located farbelow the wafer, HRVA, and auxiliary electrode, for example, 40 inchesbelow the wafer, such an electroplating apparatus, while it could bemade to function, would require quite a bit of excess power.

As was already noted, the auxiliary cathode should be relatively closeto the wafer or bottom surface of the HRVA. As way of a further example,if the auxiliary cathode was located 39 inches below the wafer with ananode 40 inches below the wafer (i.e., reasonably close to the plane ofthe anode and far from the bottom of the HRVA), most of the current fromthe anode would go to the EIRIS, but that which left the lower region ofthe electroplating apparatus anode chamber would have a great distanceto travel before reaching the wafer. Over such a distance, the currentwould tend to equilibrate back to a different current distribution bythe time it reached the HRVA and wafer, so the uniformity at the waferwould be largely unaffected by the existence of the EIRIS.Alternatively, if the anode were 0.75 inches from the wafer, 0.25 inchesbelow the HRVA, and substantially parallel to or even above the EIRIS,the electroplating apparatus also would not work as well as when theanode were substantially below the EIRIS as described above, because theEIRIS would not be as effective in removing current from the morecentral regions of the cell. Therefore, in some embodiments, thedistance of the physical anodes (or virtual anode mouth) surface closestto the wafer should be at last about 1/10 the wafer diameter below theplane of the EIRIS electrode (or virtual EIRIS cavity mouth) closest tothe wafer. For example, if the plane of the EIRIS electrodes closestpoint to a 300 mm wafer is 50 mm below the wafer and 25 mm below theHRVA, then the anode should be at least about 30 mm below that plane, ora total of 80 mm (30+50=80) below the wafer.

Structure of the Resistive Element

In certain embodiments, the ionically resistive ionically permeableelement provided herein is a microporous plate or disk having acontinuous three-dimensional network of pores (e.g., plates made ofsintered particles of ceramics or glass). The porous plates havingthree-dimensional pore networks have intertwining pores through whichionic current can travel both vertically up through the disk in thegeneral direction of the anode to wafer, as well as laterally (e.g.,from the center to the edge of the disk). Examples of suitable designsfor such plates are presented in U.S. patent application Ser. No.11/040,359 filed Jan. 20, 2005, which is herein incorporated byreference.

In other embodiments, through-holes are provided in the resistiveelement to form channels that do not substantially communicate with oneanother within the body of the element, thereby minimizing lateralmovement of ionic current in the element. Current flows in a manner thatis one-dimensional, substantially in the vector direction that is normalto the closest plated surface near the resistive element.

The ionically resistive ionically permeable element having 1-Dthrough-holes (also referred to as 1-D porous HRVA) is typically a disk(other shapes may also be used) made of an ionically resistive materialhaving a plurality of holes drilled (or otherwise made) through it. Theholes do not form communicating channels within the body of the disk andtypically extend through the disk in a direction that is substantiallynormal to the surface of the wafer. A variety of ionically resistivematerials can be used for the disk body, including but not limited topolycarbonate, polyethylene, polypropylene, polyvinylidene diflouride(PVDF), polytetrafluoroethylene, polysulphone and the like. Preferably,the disk materials are resistant to degradation in acidic electrolyteenvironment, are relatively hard, and are easy to process by machining.

In some cases, the HRVA is an ionically resistive element having a largenumber of isolated and unconnected ionically permeable through-holes(e.g., a resistive disk having multiple perforations or pores allowingfor passage of ions) in close proximity to the work piece, therebydominating or “swamping” the overall system's resistance. Whensufficiently resistive relative to the wafer sheet resistance, theelement can be made to approximate a uniform distribution currentsource. By keeping the work piece close to the resistive elementsurface, the ionic resistance from the top of the element to the surfaceis much less than the ionic path resistance from the top of the elementto the work piece edge, substantially compensating for the sheetresistance in the thin metal film and directing a significant amount ofcurrent over the center of the work piece. Some benefits and detailsassociated with using ionically resistive ionically permeable element inclose proximity of the substrate are discussed in detail in the U.S.patent application Ser. No. 11/040,359, previously incorporated byreference.

Regardless of whether the disk permits one or more dimensional currentflow, it is preferably co-extensive with the wafer, and, therefore, hasa diameter that is typically close to the diameter of the wafer that isbeing plated. Thus, for example, the disk diameter can range betweenabout 150 mm and 450 mm, with about 200 mm disk being used for a 200 mmwafer, about 300 mm disk for a 300 mm wafer, and about 450 mm disk for a450 mm wafer, and so forth. In those instances where the wafer has agenerally circular shape but has irregularities at the edge, e.g.,notches or flat regions where wafer is cut to a chord, a disk-shapedHRVA can still be used, but other compensating adjustments can be madeto the system, as described in U.S. application Ser. No. 12/291,356filed Nov. 7, 2008, naming Reid et al. as inventors, previouslyincorporated by reference. In some embodiments, the HRVA has a diameterthat is greater than the diameter of the wafer to be plated (e.g.,greater than 200 mm or 300 mm), and has an outer edge portion that ishole-free (in the case of a one-dimensional HRVA). Such edge portion canbe used to create a small gap about the periphery of the wafer (aperipheral gap between the HRVA edge portion and either the wafer edgeor the bottom of wafer-holding cup), and to assist in mounting the HRVAwithin the chamber, e.g., to an anode chamber wall. In some embodimentsthe size of the hole-free HRVA edge is between about 5 mm and about 50mm from the outer edge of the HRVA to the edge of the portion of theHRVA that has holes.

In the case of a one-dimensional HRVA, the number of through-holes madein the disk should be relatively large, but the diameter of each holeshould be quite small. Generally, the diameter of each hole generallyshould be less than about ¼ the HRVA to wafer gap. In one embodiment thenumber of holes ranges between about 6,000 and about 12,000, each hole(or at least 95% of holes) having a diameter (or other principaldimension) of less than about 1.25 mm. A schematic top view of HRVAplate 201 is shown in FIG. 2A, illustrating a top HRVA surface having alarge number of small-diameter openings, shown as black dots. FIG. 2Billustrates a cross-sectional view of the HRVA disk 201, schematicallyillustrating non-communicating through-holes. In this embodiment thethrough-holes are substantially perpendicular to the top and bottomsurfaces of the HRVA disk. The thickness of the HRVA disk ranges in someembodiments between about 5 mm and about 50 mm, e.g., between about 10mm and about 25 mm.

While HRVA shown in FIG. 2A has a uniform distribution of through-holes,in other embodiments it is advantageous to use a HRVA having regionswith non-uniform distribution of holes, or with holes that are blockedsuch that the wafer experiences non-uniform hole distribution. Such adistribution permanently directs more current to the center, so a highresistance film is more uniformly plated than if a uniform holedistribution is used. A very thick film (i.e., with a low sheetresistance), however, will tend to plate more non-uniformly if anon-uniform hole distribution is used. The blocked or missing holes maybe non-uniform in the radial, azimuthal, or both directions. In someembodiments, the ionically resistive ionically permeable element ispositioned substantially parallel to the wafer and anode surface, andthe one-dimensional through-holes are oriented parallel to the directionbetween the wafer and anode surface. In other embodiments, at least someof the holes have their relative angle modified to change the holelength relative to the element thickness, and thereby modify the localcontribution of the holes to the resistance.

It is important to note here that a HRVA is distinct from so-calleddiffuser plates, whose main function is to distribute flow ofelectrolyte, rather than to provide significant electrical resistance.As long as 1) the flow is relatively uniform, 2) the gap sufficientlylarge between the wafer holder and diffuser plane, and 3) the spacingbetween the wafer and anode is sufficiently large, the relative gapbetween a low electrical resistance diffuser and the wafer willtypically only have a minor impact on the current distribution whenplating a high sheet resistance wafer. Also, a diffuser in combinationwith an auxiliary electrode below the diffuser is not as effective inachieving uniform current, particularly on high resistance wafers, as aHRVA/auxiliary electrode combination (as described herein, and comparedin FIG. 12, curves 1203 vs. 1204), because the added voltage drop andseparation above and below the diffuser does not exist. In contrast, aHRVA significantly increases resistance of the plating system, as isneeded for improving plating uniformity.

Generally, for 300 mm systems, porous resistive elements (i.e., a HRVA)in combination with appropriate electrolytes which increase systemresistance by at least 0.45 ohm, or at least 0.6 ohm, are preferred. Theporous resistive element should increase the system resistance by atleast about 300, or preferably at least about 420 ohm cm² for each cm²of projected porous elements area. The high resistance of the providedelement is achieved by providing a low but continuously connectedporosity. It is not just the porous resistive element's porosity that isimportant, but the combined HRVA characteristic of a relatively thinlayer that has unusually low porosity, so as to achieve a compact regionof very high resistance that can be positioned in close proximity to thewafer surface. In certain embodiments, the thickness of the HRVA is lessthan about 15% of the wafer diameter. In contrast, if anon-one-dimensional, lower porosity, thicker, but still higherresistance element is used, the total system resistance could be made tobe the same, but the current diverting characteristics would not be thesame. Current within a thick, relatively low porous element tends toenter the central region and flow radially outwards as it flows upwards.Very high resistance plates tend to have the current flow only upwardstoward the wafer.

In the case of a one-dimensional HRVA, current is prevented from flowingradially by providing an extremely large number of small through-holes,each having very small principal dimension (or diameter for circularholes). For example, disks having between about 6,000-12,000perforations, with each perforation having a diameter of less than about5 mm, e.g., less than about 4 mm, less than about 3 mm, or less thanabout 1 mm are suitable resistive elements. The porosity value forsuitable disks ranges typically from 1 to 5%. Such disks increase theresistance of the plating system by about 0.3 to 1.2 ohm or more,depending on the design and electrolyte conductivity. In contrast,diffuser plates typically have openings that constitute a much largernet porosity (in the range of from 25 to 80 percent open void fraction),no more than is required to achieve a substantially uniform electrolyteflow though a significant viscous flow resistance, and generally have amuch smaller, often insignificant overall contribution to resistance ofthe plating system.

In one specific example, the ionically resistive ionically permeableelement (HRVA) is a disk having about 9,000 unique 1 dimensionalindividual perforations, each having a diameter of about 0.6 mm. Thedisk in this example has a diameter of about 300 mm (substantiallycoextensive with a 300 mm wafer), a thickness of about 13 mm, and istypically placed from about 2 to 5 mm from the wafer surface.

While a HRVA (unlike a diffuser plate) should always have substantialresistivity, in some embodiments the HRVA is configured such that itdoes not increase the system total resistance by more than about 5 ohms.While a larger system total resistance may be used, this limitation isbecause excessive resistance will require increased power to be used,leading to undesirable heating of the electroplating system. Also,because of some practical limitations of manufacturability (i.e.,creating a large number or exceedingly small diameter holes),performance (fewer holes leading to individual-hole current “imaging”),and loss of general process utility (e.g., inability to plate thickerfilms without wasted power, heat and bath degradation), about 5 ohms isa practical HRVA limitation.

Another important parameter of a one-dimensional resistive element isthe ratio of a through-hole diameter (or other principal dimension) tothe distance of the element from the wafer. It was discoveredexperimentally and subsequently verified by computer modeling that thisratio should be approximately 1 or less (e.g., less than about 0.8, orless than about 0.25). In some embodiments, this ratio is about 0.1 forproviding the best plating uniformity performance. In other words, thediameter of the through-hole should be equal to or smaller than thedistance from the resistive HRVA element to the wafer. In contrast, ifthe through-hole diameter is larger than the wafer-to-HRVA distance, thethrough-hole may leave its individual current image or “footprint” onthe plated layer above it, thereby leading to small scale non-uniformityin the plating. The hole diameter values recited above refer to thediameter of the through-hole opening measured on the HRVA face that isproximate to the wafer. In many embodiments, the through-hole diameteron both proximate and distal faces of HRVA is the same, but it isunderstood that holes can also be tapered.

The distribution of current at the wafer (and, consequently, platinguniformity) can depend on a number of factors, such as the plating gap(HRVA plate to wafer distance), electrolyte flow rate, anode chamberdesign, plating solution properties, and uniformity of hole distributionon the HRVA. Regarding hole distribution, the holes in a HRVA plate maybe designed to be of the same size and are distributed substantiallyuniformly. However, in some cases, such an arrangement can lead to acenter spike or dip in the plated film thickness, or a corrugated (wavy)pattern. Specifically, use of a HRVA having uniform distribution ofholes in the center has resulted in center spikes of about 200-300 Å for1 μm plated layer.

In one embodiment, a non-uniform distribution of 1-D pores/holes in thecentral region of the HRVA is employed to prevent the center spikes. Thecentral region of HRVA is defined by a circular region at the HRVAcenter, typically within about 1 inch radius from the center of HRVAdisk, or within about 15% of the wafer radius. The non-uniformdistribution of through-holes effective for spike reduction can have avariety of arrangements achieved by shifting holes, adding new holes,and/or blocking holes in an otherwise uniform pattern. Variousnon-uniform center hole patterns may be useful for avoiding platingnon-uniformity and are described in U.S. patent application Ser. No.12/291,356 filed Nov. 7, 2008 and previously incorporated by reference.

Virtual Electrode

Two types of current source (or sink) electrodes should be recognized ina plating apparatus as described herein: a virtual electrode and aphysical electrode. Both types of electrodes provide either currentsources (anodes) or current sinks (cathodes).

Physical electrodes are commonly known as electrochemical interfaces,typically composed of a conductive material such as a metal (e.g.copper) that are solid (or in some circumstances a liquid when using aconductive liquids such as mercury) physical structures where anelectrochemical reaction takes place at the electrolyte interface. Anexample of a physical electrode is a piece of copper where copperelectrodeposition or oxidation takes place. These physical conductiveanodes or cathodes, disposed within an electrolyte of an electroplatingchamber, can have various dimensions and can be located as desiredanywhere within an electroplating chamber, either inside or outside ananode chamber as described herein, above, below or to the side of aplating substrate or HRVA plate depending on the type of electrode andits desired function. While the physical electrode has a finite size(depth), when the electrode is non-porous (e.g. as a solid piece ofmetal), the influence of the physical electrodes on the reaction currentdistribution is generally limited primarily to the surface contour ofthe electrode exposed to the electrolyte within the chamber.

A virtual electrode has an associated physical electrode that is locatedat a position removed from that of the virtual electrode. In otherwords, the positions of the virtual electrode and its associatedphysical electrode are separated by some distance. However, the virtualelectrode is in ionically conductive communication with its associatedphysical electrode. In addition to its physical electrode, a virtualelectrode is defined by an insulating or highly resistive cavitystructure which constrains the current and current distributionassociated with the physical electrode. Such structure is typically incontact with the electroplating solution. Without the insulating orhighly ionically resistive structure, the current distribution from thephysical electrode could be significantly more non-uniform at thelocation of the virtual electrode. A typical insulating structure is afocusing tube or focusing cavity that surrounds the physical electrodein all directions except for an opening or mouth to a larger region ofthe plating chamber (e.g., an opening to the main part of the chamber).The effective location of the virtual electrode in such designs is thevirtual electrode's mouth (i.e., the position where the cavity or othercontainment structure opens into a larger region of the plating vesselsuch as the region that contains the work piece being electroplated).Examples of virtual cathodes defined by cavities in insulatingstructures are shown as elements 347 and 348 in FIG. 3A, where theassociated physical cathodes are shown as elements 340 and 342,respectively. An example of a virtual anode formed by a cavity and ahighly ionically resistive structure is the high resistance virtualanode (HRVA) shown as element 311 in FIG. 3A, and associated with anode306. Other virtual anodes are shown in FIG. 4 and the associateddiscussion in U.S. patent application Ser. No. 11/040,359, filed Jan.20, 2005, which is incorporated herein by reference for all purposes.

Often a virtual electrode can be characterized by three elements: 1) aphysical electrode, 2) a dielectric housing cavity containing ionicconductive electrolyte that confines the manner in which the ioniccurrent flows to or from the physical electrode and 3) one or morecavity mouth(s). As indicated, the dielectric housing cavity structureessentially allows one to confine, direct, and/or focus the currentdelivered to, or emanating from, the cavity though the virtual electrodecavity mouth(s). Generally, the location of the associated physicalelectrode within the virtual electrode cavity allows the physicalelectrode influence to be substantially removed from the electrode'sphysical location and transposed to the virtual electrode's location.

In certain embodiments, the physical electrode within a virtualelectrode cavity is located behind or below a membrane, such as acationic conductive membrane. Such membrane may serve the purpose oflimiting the physical electrode's exposure to plating bath additives,and/or preventing particles generated at the physical electrode fromentering the main electrode chamber or traveling to the wafer surface.In some embodiments, the mouth of the virtual electrode cavity containsa high resistance porous dielectric element (a so called high resistancevirtual anode or cathode plate). The inclusion of such a platesubstantially increases the voltage drop therein, and allows the mouthof the virtual electrode to more closely approximate a uniform currentsource, which in some cases can increase the radial effectiveness of thevirtual electrode and create a more uniform wafer current at a lowertotal auxiliary electrode current.

The non-conductive virtual electrode cavity structures (e.g. plasticwalls) direct all or substantially all of the current coming from orgoing to the physical electrode that is housed internally in the virtualelectrode cavity to emanate from or enter into the virtual electrodecavity mouth. The potential at the surface of a conductive physicalelectrode is typically approximately a single constant value. Thiscondition may be, but is not necessarily, approximated at the virtualcavity mouth. It is understood that it is not necessary for the virtualcavity mouth to have all the properties of, or result in an identicalcurrent distribution at the virtual cathode/anodes mouth location thatwould occur if a physical electrode where located there. However, allcurrent from the physical electrode must pass though the cavitymouth(s), and when the electrode, cavity, resistive element and othercomponents are appropriately designed, both the potential and currentdistribution across the virtual electrode can be made to besubstantially uniform. For example, the shape of the cavity can bemodified to improve the uniformity of plating on the physical cathode.The virtual electrode mouth region is typically, though not necessarily,planar, annular, or conical, though other shapes are certainly possible.For many purposes, the virtual electrode mouth appears to act like a“real” physical electrode because it presents a surface where currentpasses into or out of a major cell element (e.g. main anode chamber). Asindicated, this cavity mouth “surface” influences plating conditions byproviding or consuming ionic current in the similar manner that aphysical electrode would if the physical electrode were located at theposition of the virtual electrode mouth.

Electroplating System with a HRVA and an Auxiliary Cathode

Illustrations of an electroplating apparatus, which employs a resistiveelement in close proximity to the wafer (i.e., a HRVA), an auxiliarycathode, and a second auxiliary cathode are shown in FIGS. 3A-3D. FIG.3A is a diagrammatical cross-sectional view of an electroplatingapparatus. FIG. 3B is a diagrammatical cross-sectional view of anelectroplating apparatus including power supplies for a wafer and anauxiliary cathode. FIG. 3B does not show a second auxiliary cathode, asecond auxiliary cathode power supply, or any electrolyte flow paths,for sake of clarity. FIGS. 3C and 3D are further cross-sectional viewsof an electroplating apparatus. The cross-sectional views in FIGS. 3A-3Dare examples of a plating apparatus, and it is understood that theplating apparatus can be modified within the spirit and scope ofappended claims. For example, the second auxiliary cathode need not bepresent in all embodiments. As another example, the auxiliary electrodecan be below and part of the separated anolyte chamber (SAC) and haveits current pass though the SAC chamber's cationic membrane instead of aseparate membrane and different current path. As yet a further example,the HRVA need not be present in all embodiments.

Referring to FIGS. 3A-3D, a cross-sectional view of an electroplatingapparatus 302 is shown. The electroplating system comprises anelectroplating chamber that contains an anode chamber and a cathodechamber 309. The anode chamber includes two chambers, a “lower” anodechamber comprising the separated anolyte chamber (SAC) 304 where theanode 306 resides, and an upper diffusion chamber 308 (also referred toas a HRVA chamber or a catholyte chamber), separated from the separatedanolyte chamber by a cationic membrane 310. The diffusion chambercontains a highly resistive ionically permeable element (i.e., HRVA)311, described above, and an electrolyte solution (sometimes referred toas the catholyte), which is shown at a level 312. The separated anolytechamber also contains an electrolyte solution (sometimes referred to asthe anolyte), which may or may not be the same type of electrolyte inthe diffusion chamber.

The HRVA 311 is located in close proximity of the wafer (within 10 mm,preferably within 5 mm) and serves as a high resistance ionic currentsource to the wafer. The element contains a plurality of 1D throughholes and has been described in detail above.

A wafer 314 is immersed in the electrolyte solution (i.e., thecatholyte). In some embodiments, the wafer holder 316 is a clamshellapparatus which makes contacts to the periphery of the wafer through anumber of contact fingers housed behind a typically elastic “lip seal”,which serves to seal the clamshell and keep the edge contact region andwafer backside substantially free of electrolyte, as well as to avoidany plating onto the contacts. A general description of a clamshell-typeplating apparatus having aspects suitable for use with this invention isdescribed in detail in U.S. Pat. No. 6,156,167 issued to Patton et al.,and U.S. Pat. No. 6,800,187 issued to Reid et al, which are bothincorporated herein by reference for all purposes.

The clamshell is composed of two major pieces. The first piece of theclamshell is the cone, which can open allowing for insertion andextraction of the wafer. The cone also applies pressure to the contactsand the seal. The second piece of the clamshell is the wafer holdingcup. The bottom of the cup typically needs to be made of (or coatedwith) an insulator to avoid any coupled corrosion and electrodepositionreaction which would occur, for example, on a metal that is placed intoan electrolyte with a laterally varying potential, as is the case here.However, at the same time, the cup bottom needs to be mechanicallystrong. This is because it needs to be thin in order to avoidelectrolyte flow disturbances near the wafer edge while beingsufficiently strong to press the cup up against the wafer and cone whileavoiding flexing. Therefore, in some embodiments the cup bottom is metalthat is coated with an insulating material such as glass or plastic.

The cationic membrane 310 allows ionic communication between theseparated anolyte chamber and the diffusion chamber, while preventingthe particles generated at the anode from entering the proximity of thewafer and contaminating it. The cationic membrane is also useful inprohibiting non-ionic and anionic species such as bath additives frompassing though the membrane and being degraded at the anode surface, andto a lesser extent in redistributing current flow during the platingprocess and thereby improving the plating uniformity. Detaileddescriptions of suitable ionic membranes are provided in U.S. Pat. Nos.6,126,798 and 6,569,299 issued to Reid et al., both incorporated hereinby reference. A detailed description of suitable cationic membranes isprovided in U.S. patent application Ser. No. 12/337,147, entitledElectroplating Apparatus With Vented Electrolyte Manifold, filed Dec.17, 2008, incorporated herein by reference. Further detailed descriptionof suitable cationic membranes is provided in U.S. patent applicationSer. No. 61/139,178, entitled PLATING METHOD AND APPARATUS WITH MULTIPLEINTERNALLY IRRIGATED CHAMBERS, filed Dec. 19, 2008, incorporated hereinby reference.

Electrolyte solutions are continuously provided to the separated anolytechamber and the diffusion chamber by separate pumps (not shown). For theseparated anolyte chamber, electrolyte enters the chamber at lowermanifold 320 and exits at 322. For the diffusion chamber, electrolyteenters the chamber at manifold 330 and exits through 358 by flowing overweir wall 334.

Electroplating apparatus 302 also contains an auxiliary cathode 347 anda second auxiliary cathode 348. In the depicted embodiment, auxiliarycathode 347 and second auxiliary cathode 348 are virtual cathodes.Associated with auxiliary cathode 347 and second auxiliary cathode 348are physical cathodes 340 and 342, respectively. All of the embodimentsshown in FIGS. 3A-3D include virtual cathodes and associated physicalcathodes

In other embodiments, one or both of the virtual cathodes are replacedby physical cathodes, and the physical cathode is simply located at theposition of the virtual cathode. The electroplating apparatus performsin a similar manner with either virtual cathodes or physical cathodes(with no virtual cathodes). The use of virtual cathodes providesadvantages, however, as discussed below.

Auxiliary cathode 347 is located below the HRVA. It is positioned in theanode chamber (i.e., either the diffusion chamber or the sealed anodechamber). In the embodiment shown in FIGS. 3A-3D, the auxiliary cathodeis located above the cationic membrane, in the diffusion chamber. In theembodiment shown in FIGS. 3A-3D, auxiliary cathode 347 comprisesphysical cathode 340, housed in a chamber 341 with its own electrolyteflow circuit and pump (not shown). In some embodiments, the size of theauxiliary cathode (i.e., the height of the opening of the virtualcathode chamber) is about 5 to 15% (in certain embodiments, about 10%)of the radius of the wafer being plated. In FIGS. 3A-3D, electrolyteenters the auxiliary cathode chamber 341 at 350 and exits at 352. Theauxiliary cathode chamber is separated from the diffusion chamber 308 byan ion-permeable membrane 344. A rigid framework may provide support forthe membrane. The membrane 344 allows ionic communication between thediffusion chamber 308 and the auxiliary cathode chamber 341, therebyallowing the current to be diverted to the auxiliary cathode 347. Theporosity of membrane 344 is such that it does not allow particulatematerial to cross from the auxiliary cathode chamber 341 to thediffusion chamber 308 and result in wafer contamination. In someembodiments, the ion-permeable membrane 344 is a cationic membrane, suchas Nafion, and the membrane does not result in a significant ionicresistance (as compared, for example, element 349 described below).Other mechanisms for allowing fluidic and/or ionic communication betweenthe auxiliary cathode chamber and the anode chamber are within the scopeof this invention, including the ionic membranes and cationic membranesnoted above. Examples include designs in which an impermeable wall, inaddition to the membrane 344, provides some of the barrier between theelectrolyte in the anode chamber and the electrolyte in the auxiliarycathode chamber.

In some embodiments, the physical cathode 340 associated with theauxiliary cathode 347 is an annularly shaped strip of metal locatedwithin the auxiliary cathode chamber 341. The physical cathode 340 isconnected to a power supply 370 by, for example, a feed-throughconnector attached to an electrode cable (not shown). The metalcomposing the physical cathode 340 and its surface is preferably inertunder electroplating conditions. Examples on inert metals which can beused as a physical cathode include tantalum, tungsten, titanium,palladium or platinum, a palladium or platinized metal substrate such astitanium or tungsten or tantalum, iridium, iridized titanium and thelike. In some embodiments, the same material that is being plated as thephysical cathode material is used. For example, a copper-comprisingphysical cathode may be used when copper is plated.

The dimensions of the auxiliary cathode chamber 341 and of the physicalcathode 340 may vary depending on the needs of electroplating process.In some embodiments, the width of the physical cathode is about 10 to20% (about 15% in certain embodiments) of the radius of the wafer beingplated. In one embodiment, the physical cathode is a strip of metal,having a thickness of about 0.1 to 2 mm, a width of about 0.5 to 5 cm,and a length traversing the outer peripheral region of the anodechamber. Embodiments of other cathode configurations include circularbars (O-shaped toroids), C-shaped bars, coils having a circularconfiguration in which individual coils define a small circle and theoverall coiled structure surrounds the main plating vessel in theauxiliary cathode or anode chamber.

While the auxiliary cathode chamber need not be restricted to afractional volume, it is generally smaller than the anode chamber,having a volume of about 1 to 20% of the anode chamber, and in someembodiments, around 5%. As described above, it is generally desirable tohave the auxiliary cathode located relatively close to the lower surfaceof the HRVA, so that the current does not have the space in which toredistribute before reaching the wafer surface. The distance, d, betweenthe lower surface of the HRVA and the auxiliary cathode should generallybe about equal to or less than the radius, r, of wafer onto which metalis being plated (i.e., d˜≦r). In embodiments where a HRVA is notemployed, the distance, d, between the wafer and the auxiliary cathodeshould generally be about equal to or less than 1.3 times the radius, r,of the wafer onto which metal is being plated (i.e., d˜≦1.3 r). Theauxiliary cathode should also be significantly above the plane of theanode so the current from the anode has space to change directionswithout unduly large auxiliary cathode voltages or currents.

In further embodiments, a high ionically resistive porous membrane 349,generally similar in construction to that of the HRVA itself, though notrequiring particularly small or numerous holes, is positioned betweenthe auxiliary cathode chamber and the anode chamber. Such a membraneserves to shape the current distribution to the sides of theelectroplating cell, making it more uniform. A membrane for this purposetypically has between about 1 to 5% porosity. It may or may not includesmall one-dimensional holes. The resistance of the membrane 349 in thisfunction is generally commensurate with the resistance of the HRVA 311in front of the wafer, improving the current distribution uniformity tothe auxiliary electrode, as well as making the current at the virtualauxiliary electrode mouth more uniform/consistent. In certainembodiments, the high ionically resistive porous membrane 349 is lessthan about 25 mm thick, and preferably about 12.5 mm thick. Exemplaryhole diameter sizes in membrane 349 are between about 1 and 10 mm. Slotsor other openings can also be used.

In some cases, when using an auxiliary cathode located below a platingsubstrate in an electroplating apparatus, it may be desirable not to usea HRVA 311. For example, such a HRVA-free system might be sued when thewafer's sheet resistance is not greater than about 5 ohm per square. Insome cases, the auxiliary cathode alone (preferably, but notnecessarily, in combination with a second auxiliary cathode locatedabove the anode chamber and peripheral to the wafer holder, described inmore detail below) may be capable of improving the uniformity of thecurrent density experienced by the wafer to a sufficient level withoutthe additional cost and complexity of an HRVA.

The second auxiliary cathode 348 is located outside the anode chamber,outside of the HRVA-to-wafer gap 315, and outside of the peripheral gap317. As noted above, the second auxiliary cathode in the embodimentshown in FIGS. 3A-3D is a virtual cathode. The second auxiliary cathode,similar to the auxiliary cathode, has an associated second physicalcathode 342, a chamber 343 and may contain its own electrolyte flowloop, pump (not shown), and cationic membrane 346, as shown in FIGS.3A-3D. In FIGS. 3A-3D, electrolyte enters the chamber 343 at 354 andexits at 356. The cationic membrane 346 allows ionic communicationbetween the second auxiliary cathode chamber and the plating cell, whilepreventing any particles generated at the second auxiliary cathode fromentering into the plating chamber. Further details regarding theconfiguration of the second auxiliary cathode are given in U.S.application Ser. No. 12/291,356 filed Nov. 7, 2008, previouslyincorporated by reference.

In some embodiments, the second physical cathode of the second auxiliarycathode includes multiple segments, where each of the segments can beseparately powered by a separate power supply or using one power supplyhaving multiple channels adapted to independently power segments of thesecond physical cathode. Such a segmented second physical cathode isparticularly useful for plating on non-circular or asymmetrical wafers,such as wafers having flat regions. While fairly uncommon today, somewafers contain wafer “flats”, a cut out arc of the wafer at the waferedge, used, for example, for alignment. In general, however, a segmentedsecond physical cathode having independently powered segments can beused with any kind of workpiece (symmetrical or not), as it allowsfine-tuning plating uniformity. Specifically, a segmented secondphysical cathode can be used for providing current corrections atdifferent azimuthal positions of the wafer.

Because current density at the wafer flat region will generally bedifferent than the current density at the circular regions of the wafer,a different amount of current needs to be diverted from the wafer flatpart as compared from the other parts. Accordingly, in one embodiment,the second physical cathode segments are powered in concert with waferrotation, such that a first level of current is supplied to the segmentsaligned with the wafer flat region, while a second level of current issupplied to the second physical cathode segments aligned with thecircular portions of the wafer.

The second physical cathode segments can be located below, at the samelevel, or above the wafer, either in the same plating chamber as thewafer or in a different plating chamber in ionic communication with themain plating chamber. Any arrangement of the segments can be used, aslong as the segments are aligned with different azimuthal positionsabout the wafer. The number of segments can vary depending on the needsof the process. In some embodiments between about 2-10 segments areused.

While a multi-segmented second physical cathode of a second auxiliarycathode is particularly useful with a 1-D HRVA disposed in closeproximity of the wafer, as was described above, this is a separateembodiment which can be used both independently and in combination withvarious plating apparatus features disclosed herein.

Power Supplies for the Electroplating Apparatus

In certain embodiments, one or more power supplies are provided for thework piece and the one or more auxiliary cathodes. In some cases, aseparate power supply is provided for each auxiliary cathode and thework piece; this allows flexible and independent control over deliveryof power to each cathode. In the embodiment depicted in FIG. 3, three DCpower supplies are used for controlling current flow to the wafer 314,to physical cathode 340 (associated with auxiliary cathode 347), and tophysical cathode 342 (associated with second auxiliary cathode 348). InFIG. 3B, only two power supplies, one for wafer 314 and one for physicalcathode 340, are shown for the sake of clarity. The power supply 360 hasa negative output lead 362 electrically connected to wafer 114 through,e.g., one or more slip rings, brushes and/or contacts (not shown). Thepositive output lead 364 of power supply 360 is electrically connectedto an anode 306 located in the separated anode chamber 304. Similarly, apower supply 370 has a negative output lead 372 electrically connectedto the physical cathode 340, and a positive output lead 374 electricallyconnected to anode 306. Alternatively, one power supply with multipleindependently controllable electrical outlets can be used to providedifferent levels of current to the wafer and to the auxiliary cathode.The power supplies 360 and 370 can be connected to a controller 378,which allows for independent control of current and potential providedto the wafer and auxiliary cathode of the electroplating apparatus. Thesecond physical cathode (not shown in FIG. 3B) is connected to a powersupply (not shown) in a similar matter to the physical cathode.

During use, the power supplies 360 and 370 bias the wafer 314 and thephysical cathode 340, respectively, to have a negative potentialrelative to anode 306. Power supply 360 causes an electrical current toflow from anode 306 to wafer 314, plating metal onto the wafer. Powersupply 370 causes the electrical current flowing from anode 306 to wafer314 to be partially or substantially diverted to auxiliary cathode 347.The electrical circuit described above may also include one or severaldiodes (not shown) that will prevent reversal of the current flow, whensuch reversal is not desired. An undesired current feedback may occurduring plating, since the anode 306 which is set at ground potential isthe common element of both the wafer and the auxiliary circuits. A powersupply for the second auxiliary cathode operates in a similar manner.

With separate power supplies for both the auxiliary cathode and thesecond auxiliary cathode, the current applied to each of the cathodesmay be dynamically controlled. As a wafer is electroplated with metal,the sheet resistance decreases and the current non-uniformity may bereduced, making the auxiliary cathode unnecessary after a certainthickness of metal is achieved. The current supplied to the auxiliarycathode may be dynamically controlled to account for a reduction of thewafer's sheet resistance and the associated more uniform currentdistribution that normally results without the activation of theauxiliary electrode. In some embodiments, no current is supplied to theauxiliary cathode after the sheet resistance of the wafer drops to adefined level such as about 1 ohm per square or lower.

Good plating uniformity can be achieved with an appropriately designedHRVA for sheet resistances below about ½ ohm per square. Therefore, theEIRIS current can be essentially reduced to near zero below thisresistance value. More generally, if the plating process is started witha film having a sheet resistance of 100 ohm or more per square, forexample, the EIRIS current can be significantly reduced once the sheetresistance drops below about 20 ohm per square, more preferably or moreextensively when the resistance drops below 10 ohms per square. As notedabove, EIRIS current generally is not required at values below ½ ohm persquare. If the film being plated is copper, these sheet resistancesapproximately correspond to thicknesses less than 15 Å (100 ohm persquare), 50 Å (20 ohm per square), 100 Å (10 ohm per square) and 500 Å(0.5 ohm per square) of copper on the wafer

In yet further embodiments, depending on the current density applied tothe wafer, and therefore the rate of reduction of the wafer sheetresistance, no current or substantially no current is supplied to theauxiliary cathode after metal is plated onto the wafer for a setduration such as a period of about 20 seconds of less, or in otherembodiments for a period of about 5 seconds or less.

The current may be reduced to the auxiliary cathode and/or the secondauxiliary cathode simply by turning the current supplied to each cathodeoff. The current may also be constant for a period of time and thendecrease monotonically, or alternatively decrease monotonically startingfrom when the electroplating process is initiated, or from a timeshortly thereafter. The current supplied to the second auxiliary cathodemay also be dynamically controlled in a manner that is driven by and insome manner follows (e.g. is made to match proportionately) theauxiliary cathode current. One or both of the auxiliary electrodecurrents can be tied or otherwise manipulated dynamically in aproportionate manner to the total wafer current. The current supplied tothe auxiliary (and/or second auxiliary cathode) may also be dynamicallycontrolled in a manner using an algorithm calculated from and/or timeshifted from (e.g. initiation delayed until reaching a threshold triggercurrent level or time since initiation of plating) the current flowingthrough the wafer, anode or auxiliary cathode. The current supplied tothe auxiliary cathode and the second auxiliary does not need to bedecreased in the same manner or at the same rate. The current suppliedto any of the wafer, anode, auxiliary and secondary electrode may alsobe pulsed. The pulse can be simple current on/off pulses with symmetricor different duration of on and off times. Alternatively, currentforward and reverse pulses of different magnitudes and durations may beused. Control of the current supplied to one or more auxiliaryelectrodes is described in U.S. Pat. No. 6,168,693 issued to Uzoh etal., which is herein incorporated by reference in its entirety and forall purposes.

In one embodiment, the auxiliary and secondary cathodes are tiedtogether with a resistor in the line of one of them after an in-line-teesplit, the line coming from a single power supply that is used toenergize both cathodes simultaneously. In other embodiments, separatepower supplies for both the auxiliary cathode and the second auxiliarycathode are employed and allow for different current levels at differenttimes for each of the cathodes. In a specific embodiment, when currentis initially supplied to the auxiliary cathode, the ratio of the currentsupplied to the auxiliary cathode and to the substrate is at least about1:2 (i.e. half of the total wafer current), and in a further specificembodiment is at least about 5:1 (i.e. five times the total wafercurrent). The current supplied to the second auxiliary cathode istypically about 10% of the current supplied to the wafer (i.e., 1:10).Current levels for the second auxiliary cathode are described in moredetail in U.S. application Ser. No. 12/291,356, which was previouslyincorporated by reference.

An example of one possible current-time profile for an electrodepositionprocess is described below. When the electrodeposition process is firststarted for a 300 mm wafer, a 5 A current may be supplied to the wafer,a 25 A current may be supplied to the auxiliary cathode, and a 0.5 Acurrent may be supplied to the second auxiliary cathode. After a 5second time period has passed, the current supplied to the auxiliarycathode is ramped down in a linear fashion from 25 amps to 0 A over anensuing 10 second time period, while keeping a constant 5 A current and0.5 A current supplied to the wafer and the second auxiliary cathode,respectively. After a total of 20 seconds have passed, the current tothe secondary cathode is turned off (set to zero). In this case, for thefirst 5 seconds, 30.5 Amps is supplied from the anode. From 5 to 15seconds, the current from the anode decreases from 30.5 to 5.5 Amps.After 20 seconds the current to the anode drops to 5 amps and only thecurrent from the anode to the wafer remains. It is understood that thebest profile for a given circumstance depends on numerous factors suchas the initial wafer sheet resistance, the plated film specificresistivity, the plating bath conductivity, plating bath additiveinfluences, flow of the plating bath, as well as other factorsassociated with the physical cell design, so no one current-time profileis suitable for all cases. The optimum current-time profile therefore isbest determined experimentally or estimated mathematically (i.e. using acomputer model).

Controller 378 in conjunction with power supplies 360 and 370 allows forindependent control of current and potential provided to the wafer, theauxiliary cathode, and the second auxiliary cathode of theelectroplating apparatus. Thus, controller 378 is capable of controllingpower supplies 360 and 370 to generate the current profiles describedabove. The controller, however, generally is not capable ofindependently determining if one of the conditions described above(e.g., sheet resistance reaching a level of 1 ohm per square or lower)has been met, though an estimate of the sheet resistance can be madebased on a known total cumulative amount of charge passed to the waferthough lead 362 at any given time. Thus, the controller may be used inconjunction with sensors that may determine whether a condition has beenmet. Alternatively, the controller may simply be programmed with aseparate current versus time profile for each of the wafer, auxiliarycathode, and second auxiliary cathode. The controller may also measurethe charge (coulombs=integral of amperage*time) supplied to the wafer,auxiliary cathode, and second auxiliary cathode, and base thecurrent-time profile on these data.

Controller 378 may be configured to control electrical power deliveredto the auxiliary cathode in a manner that produces a more uniformcurrent distribution from the anode after electroplating a definedamount of metal onto the substrate or after electroplating for a definedperiod of time. Controller 378 may also be configured to controlelectrical power delivered to a second auxiliary cathode adapted fordiverting a portion of ionic current from an edge region of thesubstrate. Furthermore, controller 378 may be configured to ramp downelectrical power delivered to the auxiliary cathode and the secondauxiliary cathode, each at different rates, as metal is deposited on thesubstrate. Additionally, controller 378 may be configured to supply nocurrent or substantially no current to the auxiliary cathode after thesheet resistance of the substrate surface reaches about ½ ohm/square orless or metal is plated onto the substrate for a period of about 5seconds or less.

Controller 378 may also be configured to control the level of currentsupplied to the auxiliary cathode and to the substrate. In oneembodiment, the ratio of current supplied to the auxiliary cathode andthe substrate is at least about 1:2 when the current plating begins. Inanother embodiment, the ratio of current supplied to the auxiliarycathode and the substrate is at least about 5:1 when the current platingbegins.

Physical Shields

A close-up view of another configuration comprising an ionicallyresistive ionically permeable element and a second auxiliary cathode isillustrated in FIG. 4. Only the wafer 314, the second auxiliary cathode342, and the ionically resistive element 311 are illustrated to preserveclarity (i.e., elements generally found below the HRVA 311 areexcluded). In this configuration the second auxiliary cathode is locatedclose to the wafer, but the exact positioning of the second auxiliarycathode is somewhat flexible, particularly when the gap between thewafer and/or wafer holder and the HRVA plate is small and creates avirtual cathode cavity by the positional combination of these elements.The ionically resistive ionically permeable HRVA element is locatedopposite the wafer and parallel to it at a close distance d₁. Thisdistance, in one embodiment, is less than about 5 mm, or less than about3 mm. In a different embodiment, the distance is not greater than about5% (more preferably about 2%) of the diameter of the work piece'scircular region.

Another embodiment of a configuration employing an ionically resistiveelement is shown in FIG. 5. While a primary auxiliary cathode is notdepicted in FIGS. 4 and 5, it is understood that it is present at alocation below the HRVA. In this embodiment, a static insulating shield502 resides about the perimeter of the resistive element to furtherimprove edge-center plating uniformity. Optionally, the configurationalso includes a second auxiliary cathode 342. The shield may residedirectly on or slightly above the resistive element 311 and may eclipsesome of the through-holes on the periphery of the resistive element.Generally, in the lateral (x-direction) such shield can be completelyperipheral to the resistive element (outside its perimeter), partiallyperipheral (a portion of the shield is outside and a portion is insidethe perimeter, as shown), or fully on top of the outer edge of theelement (completely inside the perimeter of the resistive element).Vertically (in y-direction) the shield resides between the wafer and theresistive element, at the same level or below.

When the shield resides above the HRVA it can be used to make theperipheral gap smaller. This is particularly advantageous when a secondauxiliary cathode is used. By using different shields and the same HRVA,the relative size of the peripheral gap vs. HRVA-to-wafer gap can bemodulated independently. A separate function of the shield, according tosome embodiments, is to eclipse some holes of the HRVA, thereby blockingcurrent passing through those holes. The shield may be configured (byits size, location, dynamic movement, etc.) to change the number andlocation of open holes so as to tune current profile for a particularapplication. Advantageously, these parameters can be modulated by usingthe same HRVA and selecting a shield suitable for a particularapplication, e.g., creating a desired peripheral gap, a desired patternof blocked holes, etc. Changing the lateral extent or other dimensionsof the shield allows for manipulation of the more static (thick-film)plating uniformity of the HRVA system, and minimizes the number ofunique (an generally costly) HRVA designs and constructions, allowingone base HRVA configuration to be adapted to a wide range of uses andchanges in the plating solution (plating metal, plating additives, bathconductivities, etc.) and initial metalized wafer characteristics (seedfilm type, composition and thickness). While in some embodiments thestatic shield is a ring, as illustrated in FIG. 5, in other embodimentsthe shield can be bat-wing shaped, or have another azimuthallyasymmetric shape.

Additionally, other shields can be positioned within the plating chamberbetween the HRVA and the anode (e.g., below the HRVA inwafer-facing-down systems). The shields are usually ring-shapeddielectric inserts, which are used for shaping the current profile andimproving the uniformity of plating, such as those described in U.S.Pat. No. 6,027,631 issued to Broadbent, which is herein incorporated byreference in its entirety and for all purposes. Other shield designs andshapes may be employed as are known to those of skill in the art.

In general, the shields may take on any shape including that of wedges,bars, circles, ellipses and other geometric designs. The ring-shapedinserts may also have patterns at their inside diameter, which improvethe ability of the shields to shape the current flux in the desiredfashion. The function of the shields may differ, depending on theirposition in the plating cell. The apparatus of the present invention caninclude any of the static shields, as well as variable field shapingelements, such as those described in U.S. Pat. No. 6,402,923 issued toMayer et al. and U.S. Pat. No. 7,070,686 issued to Contolini et al.,both of which are herein incorporated by reference in their entireties.In some embodiments, variable field shaping elements placed below a HRVAcan be used in place of an EIRIS (to accomplish similar results as theEIRIS), or alternatively, in conjunction with an EIRIS. An apparatus ofembodiments of the present invention can also include any of thesegmented anodes, such as described in U.S. Pat. No. 6,497,801 issued toWoodruff et al. or concentric anodes, such as described in U.S. Pat.Nos. 6,755,954 and 6,773,571 issued to Mayer et al., all of which areherein incorporated by reference in their entireties.

The apparatus configurations described above are illustrations ofembodiments of the present invention. Those skilled in the art willappreciate that alternative plating cell configurations that include anappropriately positioned auxiliary cathode and second auxiliary cathodemay be used. While shielding inserts are useful for improving platinguniformity, in some embodiments they may not be required, or alternativeshielding configurations may be employed.

Comparison of Four Different Electroplating System Configurations

FIG. 6A-6D are cross-sectional schematic views of four differentelectroplating apparatus configurations and including a substrate havinga seed layer with a generally high sheet resistance (e.g., 1 ohm persquare or higher, or in the cases of very high resistance, 10 ohms persquare or higher). Lines of constant potential (602) and lines ofcurrent flow (604) are illustrated on the cross-sectional schematics.Also shown are representative plots of current density versus radialposition in each of the electroplating cells.

FIG. 6A illustrates an electroplating apparatus that does not include aHRVA, an auxiliary cathode, or a second auxiliary cathode. The currentdensity results are representative of the plating of a 1 ohm per squarewafer in such a configuration. As explained above, due to the highresistance to ionic current and the center of the wafer (C) and lowresistance at the edges of the wafer (R_(r) and R₁), the ionic currentdensity is low at the center and high at the edges. Such a currentdensity leads to edge-thick metal deposition profiles.

FIG. 6B illustrates an electroplating apparatus that includes a HRVA,but not an auxiliary cathode or a second auxiliary cathode. The currentdensity results are representative of the plating of a 1 ohm per squarewafer in such a configuration. The use of a HRVA makes the currentdensity largely insensitive to anode-to-HRVA spacing and the physicalcounter electrode size, as discussed further, below. As shown in thecurrent density profile, use of a HRVA provides a much more uniformcurrent density across the entire wafer. Some fraction of the current,however, still leaks at the HRVA edges, again resulting in an edge-thickdeposition profile, indicated by the increase in current in the radialregion outside the dotted line.

FIG. 6C illustrates an electroplating apparatus that includes a HRVA,but not an auxiliary cathode or a second auxiliary cathode (i.e.,identical to FIG. 6B). However, in this figure the current densityresults are representative plating performed on a seed wafer with aresistance of, for example, 10 ohm per square or higher. The use of aHRVA makes the current density largely insensitive to anode-to-HRVAspacing and the physical counter-electrode size, but the platinguniformity is much worse than for the 1 ohm per square wafer. Use of aHRVA provides a much more uniform current density across the entirewafer than if no HRVA were employed, but the plating uniformityvariation is still very significant. Similar to FIG. 6B, some fractionof the current, however, still leaks at the HRVA edges, again resultingin a particularly high near-edge-thickness deposition profile indicatedby the further increase in current in the radial region outside thedotted line.

FIG. 6D illustrates an electroplating apparatus that includes a HRVA, anauxiliary cathode, and a second auxiliary cathode, with platingperformed on a seed wafer with a resistance of, for example, 10 ohm persquare or higher. The current density profile can be modified by varyingthe sizes of various elements (including the physical counter electrodeand the electroplating cell) and the positions of various elements(including the HRVA, the auxiliary cathode, and the second auxiliarycathode). As discussed above, the current supplied to the wafer, theauxiliary cathode, and the second auxiliary cathode can also bedynamically varied. The dimensions of the HRVA and currents supplied tothe wafer, auxiliary cathode, and second auxiliary cathode can becontrolled to yield a uniform current density across the entire wafer(as shown in the current density plot), resulting in flat metaldeposition profile, even when the substrate sheet resistance is veryhigh (typically during the initial few seconds of deposition).

It should be understood that the result obtained with the apparatus ofFIG. 6D may be achieved not only for films whose sheet resistances arethe same as in FIG. 6A and FIG. 6B (about 1 ohm per square or lower),but also for films with sheet resistances equal to and greater than thesheet resistances of the film for FIG. 6C (greater than about 10 ohm persquare). The general utility of the method and apparatus of thisinvention can be appreciated by the fact that in addition to uniformplating on very high resistance wafers, when not utilizing (energizing)the auxiliary electrode of the apparatus of FIG. 6D, the uniform currentdistribution observed for a relatively low sheet resistance wafer (lessthan about 1 ohm per square) may also be obtained, as can be obtainedfrom the apparatus of FIG. 6B, or with low sheet resistances (less thanabout 0.2 ohm per square) from the apparatus of FIG. 6A. Therefore, theapparatus of FIG. 6D has a range of applicability in terms of uniformplating on both very high and very low sheet resistance wafers, with theability to plate with at least equally good uniformity on wafers withrelatively low sheet resistances that might be obtained from apparatusof FIG. 6A or 6B.

Positioning and Shape of an Anode

In the presence of a HRVA, which serves as a virtual current source, thepositioning of the physical (metal) anode (a positively biased member ofthe plating chamber) may be relatively unimportant, or certainly lessimportant than in the absence of a HRVA in the system. Therefore, whilein some embodiments the actual anode (a positively biased electrode) isaligned with the wafer and with the resistive element and is disposedopposite to the wafer surface, in other embodiments, the anode is notaligned with the wafer, as it may be shifted to the side with respect tothe wafer, be placed in a separate chamber, or may have dimensions(e.g., diameter) substantially different from those of the wafer. Insome embodiments, the anode is disk-shaped and is aligned with thewafer. In other embodiments the anode may be positioned asymmetrically.In yet other embodiments, multiple anodes are used. In still otherembodiments, the anode is positioned remotely from the chamber orhousing inside an ancillary virtual anode chamber with insulatingchamber walls.

Process Flow Embodiment

An example of an electroplating method in an apparatus equipped with aHRVA, an auxiliary cathode, and a second auxiliary cathode isillustrated in the process flow diagram of FIG. 7. In the depictedembodiment, the process starts in an operation 701 by placing orreceiving the wafer into a wafer holder of the electroplating apparatus.The wafer can be placed in a horizontal or in a tilted orientation withrespect to the plane of electrolyte in the bath.

Next, in an operation 703, the wafer is put into contact with thecatholyte and is placed horizontally and parallel to HRVA, preferablywithin about 5 mm or less of HRVA, where the distance refers to thedistance between the bottom wafer surface immersed in electrolyte andthe proximate HRVA surface. After the wafer is positioned in theelectrolyte (or while the wafer is being immersed), in operation 705,current is supplied to the wafer in order to plate metal onto theseed/barrier layer. Such current is provided by controlling the currentand/or the potential of the wafer. A reference electrode can be used tocontrol the potential between the wafer and the reference electrode,preferably located in the region above the HRVA and below or to the sideof the wafer. Current (typically, but not limited being cathodiccurrent) is also supplied to the auxiliary cathode and the secondauxiliary cathode in operation 705 to shape the current distributionfrom the anode such that current density is relatively flat at the wafersurface. Anodic current from the either electrode will tend to have thereverse effect on the plating uniformity than cathodic current, tendingto increase the amount of plating at the edge or near edge. The wafer,auxiliary cathode and second auxiliary cathode are all connected via oneor more power supplies, each with one or more power supply channels, toan anode. In a particular embodiment, each of these is connectedindividually to the same single anode, located within an anode chamber.In other embodiments, the various auxiliary cathodes may be connected tosome of the same anodes, and some anodes which other cathodes or thewafer are not connected to.

In operation 707, current supplied to the wafer, the auxiliary cathode,and the second auxiliary cathode is dynamically controlled. The currentsupplied to the auxiliary cathode and, optionally, to the secondauxiliary cathode is decreased such that no current or substantially nocurrent is supplied to these cathodes after a condition is met. Theseconditions include, e.g., the sheet resistance of the wafer reachingabout 1 ohm per square or lower, at least about 100 angstrom of metalbeing plated onto the wafer, or a period of time elapsing, as discussedabove. Metal is then plated onto the wafer in operation 709 until thedesired metal thickness is reached.

Note that in some embodiments, the deposition may be divided into filland overburden phases. Typically, the current increases significantlyduring the overburden phase, sometimes by about 5 to 10 times. Forexample, if a 300 millimeter wafer receives about 5 A of current duringthe fill phase, it may receive about 30 to 40 A during the overburdenphase. Further adjustments may be made to the current supplied to theauxiliary cathode and the second auxiliary cathode during the overburdendeposition to aid in electromechanical polishing. Such adjustments mayinclude creating a non-uniform current density that is experienced bythe wafer.

Experimental Results

FIGS. 8A and 8B show the post-plating sheet resistance and calculatedfilm thickness (using the known thin film thickness-dependent specificresistivity and the measured sheet resistance) of a wafers, having aninitial sheet resistance of 50 ohm per square, plated with copper. Line801 is the final sheet resistance of the plated film, and line 803 isthe final calculated plated film thickness of the wafer processed in acell in accordance with embodiments of this invention, the cell havingboth a high ionic resistance membrane (HRVA) and an auxiliary electrodewithin and above an anode in an anode chamber. The HRVA employed hadabout 9000 individual 0.026″ diameter 1-dimensional holes that resultedin a total plate porosity of about 5%. The ionically resistive ionicallypermeable HRVA element was 12.7 mm thick and the front surface of thewafer was positioned 3.5 mm from the HRVA top surface. A ring-shapedEIRIS auxiliary electrode, 20 mm in height and about 1 mm in thicknesswas positioned inside a peripheral cavity of the anode chamber. Theperipheral virtual electrode cavity, within which the above physicalelectrode was housed, had a depth of about 10 mm (electrode on theouter/back surface), an average height of 20 mm, and walls thatrestricted the flow of current from the physical electrode to thevirtual electrode mouth connecting to the anode chamber parallel to thewafer plane. The virtual electrode “mouth” was between 10 and 30 mmbelow the HRVA bottom face. The anode was positioned at the bottom ofthe anode chamber or cavity 115 mm below the HRVA bottom face.

The total wafer current, current density, charge and plating timeapplied to the wafer was 2 amps, 2.8 mA/cm² 28 coulombs, and 14 seconds,respectively. The current applied to the auxiliary electrode was aconstant 10 A, also for 14 seconds. As can be seen from the flat,non-radial dependent sheet resistance and film thickness, a uniformcopper film was deposited. The center thickness is about 135 Å and thethickness at 125 mm from the edge is about 125 Å. At the very edge,beyond about 135 mm, the sheet resistance tends to be very low and thethickness becomes quite large as current leaks from the edge of the HRVAto reach the wafer edges. However, a secondary cathode, peripheral tothe wafer and above the HRVA as described above, was not employed inthis test, and had it been, this edge current would have beeneffectively reduced, enabling a uniform current distribution right tothe edge of the wafer.

In a contrasting experiment demonstrating the value of adding the EIRISelectrode to this system, all the physical and process parameters of thewafer, the ionically resistive ionically permeable HRVA element, andvarious cell element physical spacings were kept constant, but theauxiliary electrode was not energized (i.e., no current was applied).Curves 802 and 804 are the corresponding post-plating sheet resistanceand calculated film thicknesses for this case, showing a verynon-uniform plating thickness across the entire wafer surface. Thecenter thickness in this case is about 45 Å and the thickness at 125 mmis over 150 Å.

In some cases, where the auxiliary electrode is not employed, or it isnot employed in combination with a HRVA, and/or other unfavorableprocessing conditions are present (e.g., various current densities, bathproperties such as conductivity, or wafer sheet wafer resistance), noplating occurs in the center of the wafer at all and the centerthickness is found to be almost zero, or the wafer metal may evencorrode in the center. This phenomenon is possibly associated with theunusually large and uncompensated terminal effect creating conditionswhere plating (i) can not overcome the nucleation activation energy orpotential to start the process, or (ii) is so small that backgroundcorrosion dominates there.

FIG. 9 is a similar comparison of FIG. 8A for an ionically resistiveionically permeable HRVA element vs. a HRVA and EIRIS auxiliary-cathodecombined system, but for a less resistive 10 ohm per square seed film.The physical arrangement and the applied current of this test wassimilar to that described for FIG. 8, except as noted hereafter. Curve901 is the result with a 2 A current applied to the wafer and 4 ampsapplied to the auxiliary electrode, both for 14 seconds. No secondarycathode was employed for the test corresponding to Curve 901. As can beseen for curve 901, the sheet resistance is very flat with radius,except out to the very edge beyond about 135 mm, where the sheetresistance increases associated with a thicker film and no secondarycathode compensation, similar to curve 801. Curve 902 is a comparativeexperiment, where the EIRIS auxiliary electrode was not used orenergized (0 amps applied), but a second auxiliary cathode was energizedfor 14 seconds at a current of 0.2 A. The sheet resistance of curve 902is very high in the center and non-uniform across the wafer. Because ofthe utilization of the second auxiliary cathode, the edge sheetresistance does not continue to decrease. In fact, the sheet resistanceactually increases at the edge, which is due to the fact that a secondauxiliary cathode with an applied current of less than 0.2 A orapplication of the current for a shorter time would have been optimal.

FIG. 10 shows a 5 ohm per square PVD sputtered copper seeded waferplated in the same cell configuration as described with respect to FIG.8, but in this case having and employing the an ionically resistiveionically permeable HRVA element, auxiliary and second auxiliarycathodes. The initial sheet resistance profile 1001 is reasonablyuniform. The wafer was plated at 2 A for 28 seconds in a copper platingbath. During the first 14 seconds, a current of 4 amps was applied tothe auxiliary electrode and a current of 0.2 amps was applied to thesecondary cathode. For the final 14 seconds, no current was applied toeither the auxiliary or secondary electrodes. As can been seen, a finalsheet resistance of about 0.8 ohms per square over the entire surfacewas achieved.

Numerical Modeling

To further improve the understanding of the interactions of the systemcomponents, finite element numerical modeling using the commercialsoftware FlexPDE™ was performed to compare the various plating systems(i.e., with or without an ionically resistive ionically permeable HRVAelement and with or without an auxiliary electrode). The HRVA physicalproperties used in the simulations were the same as that described forFIG. 8. The mesh and layout of the simulation are shown in FIG. 11. Thewafer 1101 lies above the HRVA 1103 and is separated from the HRVA topsurface by a gap 1104. The wafer is held in an insulating holder,outside the wafer surface 1108. The second auxiliary cathode physicalelectrode 1109 is outside the gap created by the wafer holder 1108 andthe material peripheral to the HRVA, but was not energized in any of thesimulation results presented here. The anode 1102 resides at the bottomof the cell. For this simulation, the anode 1102 and the virtualauxiliary cathode 1106, which contains the physical auxiliary cathode1105, all reside below a cationic upper to lower chamber separatingmembrane 1107. Because the specific conductivity of this layer (i.e.,the membrane) is close to that of the electrolyte (better than 1/10^(th)that of the electrolyte), and it is so thin (less than about 0.5 mm),its presence does not appreciable change the system's total resistanceor response appreciably. However, in practical operations, theelectrolyte in the region above the membrane versus that below it may bedifferent, and the inclusion allows for different conductivities of theregions in the model. For simplicity, the data shown here used the sameelectrolyte conductivity though the cell. That is, the conductivity inthe wafer to HRVA gap, in the 1-dimensional HRVA holes, and in the restof the cell was equal to that of a typical copper plating electrolyte(e.g., a 10 g/L sulfuric acid, 40 g/L copper ion plating solution),about 6 ohm⁻¹ m⁻¹.

FIG. 12 shows the results of simulations of the initial current densityon a 50 Å seed wafer with the physical layout and with properties asdepicted and discussed with reference to FIG. 11. In addition, the waferwas set to a ground potential (zero volts), the anode at 50V, and thevoltage on the auxiliary physical electrode to 18V. Curve 1201 is thesimulation result when the cell had no HRVA and no auxiliary electrodecurrent (the electrode was removed from the system, but the virtualelectrode cavity remained). The current density is non-uniform center toedge. The center current is very low, and the edge current very high,with a monotonic transition between the center to edge, with a greaterthan 14 fold difference between the center current density to currentdensity at 125 mm. Curve 1202 had the HRVA added to the system, but notthe auxiliary electrode. The shape of the curve is generally similar tocurve 1201, though the center current is markedly higher, and the edgecurrent much lower. The current at 125 mm is now only about 3 times thatof the center current density. Still, for this high sheet resistancefilm, such a result would be unsatisfactory.

Curve 1203 shows the results obtained if the auxiliary cathode wasenergized and the HRVA was removed from the system. The uniformity ismarginally improved with respect to the HRVA only system, though themonotonic shape of the curve is lost. From the center to the about 40mm, like in all other cases, the distribution is quite flat, thoughhere, the current density is higher than in curve 1201 and slightlyhigher than in 1202. However, curve 1203 has a double plateau shape,with a second region of approximately constant current density from aradius of about 60 mm to 110 mm, before finally increasing quite rapidlybeyond this radius. The total difference in current density between thecenter and 125 mm is greater than 2×, a slight improvement over curve1202. This result indicates the significant benefit of using anauxiliary cathode chamber above the anode, even without also using anHRVA. However, current distributions that are bimodal or have anon-monotonic multiple plateau distribution are generally harder tomatch and process by subsequent metal removal processes (e.g. CMP), sothe reduction in range at the expense of the curve shape may not be asbeneficial as simply obtaining a relatively smooth monotonic profilewith a slightly greater range.

Finally, curve 1204 shows the case where both a HRVA and an auxiliaryelectrode were employed. In agreement with the experimental data, thedistribution is flat all the way from center to the edge of the wafer,varying by less than 3%.

CONCLUSION

It is understood that the examples and embodiments described herein arefor illustrative purposes only and that various modifications or changesin light thereof will be suggested to persons skilled in the art.Although various details have been omitted for clarity's sake, variousdesign alternatives may be implemented. Therefore, the present examplesare to be considered as illustrative and not restrictive, and theinvention is not to be limited to the details given herein, but may bemodified within the scope of the appended claims. Further it isunderstood that many features presented in this application can bepracticed separately as well as in any suitable combination with eachother, as will be understood by one of skill in the art.

What is claimed is:
 1. An apparatus for electroplating metal onto asubstrate, the apparatus comprising: (a) a plating chamber configured tocontain an electrolyte and an anode while electroplating metal onto thesubstrate; (b) a substrate holder configured to hold the substrate suchthat a plating face of the substrate is positioned at a defined distancefrom the anode during electroplating, the substrate holder having one ormore electrical power contacts positioned around a substantiallycircular perimeter and arranged to contact an edge of the substrate andprovide electrical current to the substrate during electroplating; (c)an ionically resistive ionically permeable element shaped and positionedbetween the substrate and the anode during electroplating, the ionicallyresistive ionically permeable element having a substantially flat uppersurface that is substantially parallel to and separated from a platingface of the substrate by a gap of about 5 millimeters or less duringelectroplating, wherein the ionically resistive ionically permeableelement has an ionically resistive body with a plurality of perforationsmade in the body such that the perforations do not form communicatingchannels within the body and wherein said perforations allow fortransport of ions through the element; (d) a first auxiliary cathodelocated between the anode and the ionically resistive ionicallypermeable element, and peripherally oriented to shape the currentdistribution from the anode, while the first auxiliary cathode issupplied with current during electroplating; and (e) a second auxiliarycathode positioned to be located between the ionically resistiveionically permeable element and the substrate during electroplating,wherein the second auxiliary cathode is adapted to divert a portion ofionic current from an edge region of the substrate.
 2. The apparatus ofclaim 1, wherein substantially all said perforations in the body of theionically resistive ionically permeable element have a principaldimension or a diameter of the opening on the surface of the elementfacing the surface of the substrate of no greater than about 5millimeters.
 3. The apparatus of claim 1, wherein the ionicallyresistive ionically permeable element is a disk having between about6,000-12,000 perforations.
 4. The apparatus of claim 1, wherein theionically resistive ionically permeable element has a porosity of about5% or less.
 5. The apparatus of claim 1, wherein the second auxiliarycathode is located in substantially the same plane as the substrate,during electroplating.
 6. The apparatus of claim 5, wherein the secondauxiliary cathode is located peripheral to the substrate holder andradially outward of a peripheral gap between the ionically resistiveionically permeable element and the substrate holder.
 7. The apparatusof claim 1, wherein the second auxiliary cathode is a virtual auxiliarycathode having an associated physical cathode housed in a cavity in theplating chamber, wherein the cavity is in ionic communication with theplating chamber.
 8. The apparatus of claim 1, further comprising asecond ionically resistive ionically permeable element, wherein thesecond ionically resistive ionically permeable element is positionedproximate the first auxiliary cathode.
 9. The apparatus of claim 8,wherein the second ionically resistive ionically permeable element hasan ionically resistive body with a plurality of perforations made in thebody such that the perforations do not form communicating channelswithin the body, wherein said perforations allow for transport of ionsthrough the element, and wherein substantially all perforations have aprincipal dimension or a diameter of the opening on the surface of theelement facing the interior of the plating chamber of no greater thanabout 10 millimeters.
 10. The apparatus of claim 8, wherein the secondionically resistive ionically permeable element has a porosity of about5% or less.
 11. The apparatus of claim 8, wherein the second ionicallyresistive ionically permeable element is less than about 25 mm thick.12. The apparatus of claim 1, further comprising a control circuitdesigned or configured to control electrical power delivered to thefirst auxiliary cathode such that the power is delivered only in thebeginning of electroplating, but not for the entire time ofelectroplating the substrate.
 13. The apparatus of claim 12, wherein thecontrol circuit is designed or configured to ramp down electrical powerdelivered to the first auxiliary cathode and the second auxiliarycathode, each at different rates, as metal is deposited on thesubstrate.
 14. The apparatus of claim 12, wherein the control circuit isdesigned or configured to supply no current or substantially no currentto the first auxiliary cathode after the sheet resistance of thesubstrate surface reaches about 1 ohm/square or less.
 15. The apparatusof claim 12, wherein the control circuit is designed or configured tosupply no current or substantially no current to the first auxiliarycathode after metal is plated onto the substrate for a period of about 5seconds or less.
 16. The apparatus of claim 15, wherein the controlcircuit is programmed with current-time instructions to supply nocurrent or substantially no current to the first auxiliary cathode aftermetal is plated onto the substrate for a period of about 5 seconds orless.
 17. The apparatus of claim 12, wherein the control circuit isdesigned or configured to supply current to the first auxiliary cathodeand the substrate at a ratio of at least about 1:2 when current platingbegins.
 18. The apparatus of claim 12, wherein the control circuit isdesigned or configured to supply current to the first auxiliary cathodeand the substrate at a ratio of at least about 5:1 when current platingbegins.
 19. The apparatus of claim 1, wherein the distance between thefirst auxiliary cathode and a lower surface of the ionically resistiveionically permeable element is less than about a radius of thesubstantially circular perimeter around which the one or more electricalpower contacts are positioned.